Temozolomide is a second generation imidazotetrazine. This study aims to 1) determine the maximun tolerable dose of oral temozolomide given daily for 5 days, 2) determine the toxicities of temozolomide given on this schedule, 3) determine the pharmacokinetic behavior of temozolomide administered orally for five consecutive days in pediatric and adolescent patients, and 4) to determine the antitumor activity of temozolomide within the confines of a Phase I study.